selftests/tc-testing: add selftests for cbq qdisc

Test 3460: Create CBQ with default setting
Test 0592: Create CBQ with mpu
Test 4684: Create CBQ with valid cell num
Test 4345: Create CBQ with invalid cell num
Test 4525: Create CBQ with valid ewma
Test 6784: Create CBQ with invalid ewma
Test 5468: Delete CBQ with handle
Test 492a: Show CBQ class

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>